1. Field of Invention
The present invention relates to a tunnel boring machine, and more particularly to a cutting arrangement for a tunnel boring machine which is capable of providing a shield with varying diameters.
2. Description of Related Arts
According to the definition used by the International Society for Rock Mechanics, soft rocks are defined as rocks having low mechanical strength, high porosity, a low degree of cementation, and are substantially affected by wind erosion, or those having a large amount of expansive clay minerals. Such rocks usually consist of mudstone, shale, siltstone, sandstone, or any other naturally formed rocks having an axial compressive strength of less than 25 MPa.
Tunnel boring machines used in soft ground are called soft ground tunnel boring machines. Conventional tunnel boring machines usually comprises a rotating cutting head and a plurality of cutting tools securely provided on the cutting head. Conventional tunnel boring machines may bore tunnels in various diameters. The resulting tunnel diameter depends on a cutting diameter of the rotating cutting head and operation and the dimensions of the cutting tools. Conventionally, for almost all tunnel boring machines, the rotating cutting head and the cutting tools are manufactured to a predetermined size and configuration so that each tunnel boring machine can only used for boring tunnel having a fixed predetermined tunnel diameter. The disadvantage of this limitation is that engineers and technicians are unable to alter the size of the rotating cutting head and the positions of the cutting tools on the rotating cutting head for altering the diameter of resulting bored tunnel. In other words, for conventional tunnel boring machines, the only way of boring tunnel of various diameters is to require engineers or technicians to enter the tunnel and change the cutting tools. Obviously, this practice is very dangerous to the engineers and the technicians concerned and substantially increases construction costs and time of the relevant tunnel. In some situations, engineers or technicians are unable to enter the tunnel or perform changing procedure on the rotating cutting machine. In these situations, the diameter of the tunnel cannot be altered according to environmental or to engineering circumstances and this substantially affect the quality of the construction work.